The second major gay rights case at the Supreme Court involves a challenge to the federal Defense of Marriage Act of 1996.
The law restricts the receipt of more than 1,100 federal benefits to man-woman marriages. In essence, DOMA bars same-sex married spouses from obtaining the same federal benefits received by heterosexual married spouses.
Same-sex couples argue that the federal restriction violates their right to equal treatment.
The issue arises in the case of New York resident Edith Windsor, who says she was wrongly denied a marital exemption from the federal estate tax because of her same-sex marriage.
Ms. Windsor and Thea Spyer lived together for 44 years and were formally married in Canada in 2007. Ms. Spyer died two years later.
Although their marriage was recognized as legal in their home state of New York, under DOMA the Internal Revenue Service did not consider their same-sex relationship a marriage. Without the marital exemption to the federal estate tax, Windsor owed $363,000 in federal estate taxes.
If the couple had been a man and a woman rather than two women, Windsor would have owed no federal tax.
A federal judge agreed with Windsor, and ruled that DOMA violated her constitutional rights. The Second US Circuit Court of Appeals in New York, in a 2-to-1 decision, went even further. It found that gays and lesbians are entitled to a higher level of legal protection. Under that standard, DOMA must be struck down, the court said.
In his brief urging the court to overturn that decision, Washington lawyer Paul Clement says that Congress has the power to define marriage as part of a uniform system of distributing federal benefits and that the law is not discriminatory.
"DOMA does not bar or invalidate any state law marriage, but leaves states free to decide whether they will recognize same-sex marriages," Mr. Clement writes. "DOMA simply asserts the federal government's right as a separate sovereign to provide its own definition for purposes of its own federal programs and funding."
Lawyers for Windsor say that when states allow same-sex couples to marry, the federal government must recognize those unions as legal marriages and allow equal access to federal benefits for same-sex spouses.
"The question presented here is a narrow one: is there a sufficient federal interest in treating married gay couples differently from all other married couples for all purposes under federal law? There is not," Roberta Kaplan of New York writes in her brief urging the court to declare DOMA unconstitutional.